Best frozen foods in my experience are Brine & Mysis Shrimp, Blackworm and Bloodworm. They will eat Krill, though mine were never fans of it, possibly due to the size as cchhcc has said.
I feed frozen food about twice a week, NLS staple otherwise. I quite often crush up 1mm pellets to feed as a powder to get them turning over the sand. They'll also become more active and personable once your sand has "aged" a little. Mine were quite shy after moving them to their new setup, but they're finally starting to actively sift throughout the day like they used to now that the sand isn't "fresh".